SALPIDAE FORMS WITH COVERING FOLDS. 



443: 



begins to assume a simpler form (Fig. "219 , n) at still later stages- 

 it is found as a cell-mass running obliquely downwards and forwards ; 

 within this muss a cavil v appears which communicates anteriorly with 

 the respiratory cavity (Fig. 222). The part lying nearest to the aper- 

 ture of the central canal represents the rudiment of the ciliated pit 

 (/), while the blind end that is directed backward and upward forms 

 the ganglion proper (n). In the course of development these two- 

 sections of the neural rudiment become more sharply marked off from 

 one another by a constriction. At the same time, in the ganglionic 

 part of the rudiment, the central canal becomes segmented, breaking 

 up into three consecutive cerebral vesicles, a condition first noticed 

 by KOWALEVSKY, and similar to that 

 seen in .V. (tcmwratioa (p. 429). In 

 later stages the cerebral rudiment 

 becomes completely separated from 

 the ciliated pit, and the two rudi- 

 ments shift apart, although they seem 

 still to remain connected by a nerve- 

 strand that runs forward from the 

 brain to the ciliated pit. The central 



cavity of the cerebral rudiment disap- 

 pears, and the interior of the rudiment 



then seems filled with punctate nervous 



tissue (LEYDIG'S " Punktsubstan/ "). 



A process running towards the surface 



leads to the development of the eyes, 



in which single cells of the ganglion 



become changed into elements sensi- 

 tive to light, while other cells of the 



most superficial layer become filled 



with pigment (p. 430). No details 



are known of the development of the 



paired auditory vesicles which lie in 



contact with the brain, and were first 



observed by H. MULLER and further described by TODARO (No.. 



107). The ciliated pit, by the folding of its walls, assumes a com- 

 plicated form approaching that of the same organ in the adult (Fig. 



314,3,/). 



The pericardial rudiment (Figs. 218, 219 pc), which is originally 



a cell-strand running from above downward, divides into two parallel 



strands (Figs. 223, 224 A) ; the anterior strand, near the enteric 



FIG. 223. Horizontal section 

 through an embryo of Salpa 

 nuinata (after SALENSKY). b r 

 blastomeres ; d, enteric rudi- 

 ment ; /n, lue.senchyme-cells ; n, 

 rudiment of the nervous system ; 

 nh, neural cavity; pc, pericardial 

 rudiment ; x, slib - pericardial 

 cell-strand ; z, cells in the lumen 

 of the pharynx (atrium, BROOKS).. 



